To use the compiler without typing the complete path, you can create symbolic links in the /usr/bin/ directory for the specific invocations that are contained in the installation_path/vacpp/bg/12.1/bin/ directory.
If you have not already done so when you ran xlc_install, you can create the symbolic links for the following compiler invocations:
ln –s installation_path/vacpp/bg/12.1/bin/invocation /usr/bin/invocation